The present process relates to an improvement for obtaining a uniform air-free paste containing a dispersion or solution of substantial amounts of dry insoluble materials in a liquid phase, and more specifically for obtaining an essentially gas-free toothpaste or dental cream.
Dentifrices, such as toothpaste or dental creams are generally extrudable pastes containing insoluble polishing agents and/or abrasives that aid in the removal of plaque, stains and other deposits from the teeth and help to polish them. Due to the physical properties of toothpaste formulations and the nature of the ingredients contained therein, toothpastes will contain a certain amount of entrained air unless suitably prepared. The presence of entrained air in these types of dentifrice formulations generally result in objectionable features in the final product. For example, if a significant amount of entrained air is present the product will not have a homogeneous consistency and appearance, and if a severe and varying amount of air is involved, differences in the final weight for a given container will manifest itself. In the case of translucent toothpastes, the presence of small amounts of air will cause the paste to become opaque.
Air becomes entrained in toothpaste products because of the ingredients and methods that are employed to make up the various formulations. Toothpaste or dental cream formulations contain a variety of constituents, the major class of constituents generally being an insoluble powdered polishing agent and/or abrasive, a bodying agent, a liquid vehicle and a gum stabilizer and/or gelling agent. Other ingredients include flavoring agents and various other constituents for cosmetic, therapeutic or aesthetic effects. The polishing agents or abrasives are generally finely divided water-isoluble powdered materials whose particle size will usually pass through a 140 mesh screen, U.S. Standard Sieve Series.
Toothpastes containing the above-identified class or ingredients can be made up by various and many well-known techniques. One such technique is disclosed in U.S. Pat. No. 3,840,657 issued Oct. 8, 1974 wherein a toothpaste composition is manufactured by making a mixture of a liquid vehicle, gelling agent and polishing agent; degassing this mixture; preparing a second mixture of synthetic organic detergent and liquid vehicle therefor, and degassing this mixture by raising it to an elevated temperature; and finally admixing the first and second mixtures. In the manufacture of toothpastes containing the class of compounds noted above, however, invariably the powdered constituents must be mixed and dispersed with the other liquid ingredients contained in the final toothpaste product. Care must be taken to remove the air in the mixing in of the powders so as to prevent the entrainment of air in the resulting paste.
Various known methods are disclosed in such standard references as "Cosmetics: Science and Technology", by Sagarin, Volume I, pages 510-511, published by Interscience Publishers, Inc. (1972). One such method is the mixing of a paste mass under vacuum to remove the entrained air. The speed of removal will generally depend on the air quantity and mixer construction. Air can also be removed by atmospheric mixing followed by the use of a continuous deaerator such as a Versator available from Cornell Machine Co. The efficiency of deaeration will again be a function of the quantity of air present in the paste mass.
Numerous pieces of equipment are available and capable of achieving a dispersion of solids in liquids. These are essentially mixers containing high speed blades or discs such as a Cowles Dispersall. However, this type of equipment incorporates large quantities of air into the powder-liquid mix. In addition, if the resulting paste mass becomes too thick, insufficient movement occurs and the blade becomes ineffective.
Combination mixers such as an Abbe Dispersall or Eppenbath Agi mixer combine slow speed gate type agitation with a high speed dispersing unit to overcome the above-noted problem of thickened pastes, but still incorporate large quantities of air. Horizontal high speed mixers such as the Day Turbulizer and the Littleford are also effective in wetting down powders, but again, incorporate large quantities of air.
The current practice, therefore, for inhibiting the incorporation of air as the powders are mixed with the liquid constituents, has been to add both liquids then solids to a vacuum mixer followed by sealing of the mixer, evacuating or drawing a vacuum on the contents of the mixer, and then intimately mixing the ingredients to a homogeneous paste. Unfortunately, this procedure has several disadvantages. Generally the full working capacity of the vaccum mixer cannot be utilized since the mixing chamber must contain all of the unwet powders and liquids and be sealed prior to evacation. It will be understood that the volume occupied by the unwetted powders plus liquids invariably exceeds the volume of the resulting finished paste. This is particularly true in toothpaste manufacture where the solids represent 10-60% of the total product and wherein the powders are low in bulk density. The result is that as the powders are added to the mixing chamber, they will have a tendency to remain on top of the liquids, and significant powder losses to the vacuum system can occur. Generally, these low-bulk density powders such as those used in translucent dentifrices must be forced down into the liquids.
Alternatively, if the full working capacity of a vacuum mixer is to be utilized the powders must be added stepwise because the volume of the unwetted powders plus liquids cannot at any time exceed the capacity of the mixing chamber. Such stepwise blending is time consuming and can contribute to powder losses to the vacuum system.
Another problem involved in the wetting out of paste solids using known techniques is that as the powders are taken up in the mixing tank, there is a strong tendency towards lumping when combined with the liquids present therein. Again, if high speed atmospheric mixing is employed, large quantities of air are entrained. On the other hand, if high speed vacuum mixing is employed sufficient headspace must be made available in the mixer to contain all of the unwet powders plus liquids so that the mixer can be sealed before vacuum is drawn and mixing initiated. All of these alternatives involve either excessive entrainment of air, a reduction in mixer capacity or a time-consuming stepwise process, depending on the method used. In the case of the two latter alternatives, powder can be lost to the vacuum system.
In accordance with the present invention, therefore, a method is provided for the rapid wetting and dispersing of difficult to wet insoluble powders or solids for the preparation of paste-type formulations, particulary toothpaste formulations wherein the powders constitute a major portion of the product, while avoiding air entrainment therein. An essentially gas-free paste is obtained by the present method and is a marked simplification of earlier techniques. The method comprises charging the paste liquids to a mixing tank; subjecting the paste liquids to a continuous vacuum and intimate mixing; and introducing the paste powders or solids directly into the paste liquids through the bottom of said mixing tank while maintaining said vacuum and intimate mixing; thereby forming an essentially gas-free paste. It is critical to the process that the vacuum maintained on the mixing tank or paste liquids be of a sufficient level to draw the respective powders or solids into the paste liquids at the bottom of the tank. In effect, the vacuum imposed on the paste liquids is used as the motivating force to convey the powders or solids into the bottom of the mixing tank, thereby creating a wet trap which continuously separates the air from the powders as they are wetted. It will be appreciated that if the vacuum is not continuously maintained, the powders or solids will than have to be added in several stages. Thus, as the powders or solids are pulled into the mixing tank, they are accompanied by entrained air that must constantly be pulled off. This is accomplished by exposing the air in the mixing operation to the constant vacuum and turbulence created by the intimate mixing. Removal of the air will be more efficiently obtained and the powders most effectively dispersed if the powders are introduced into the paste liquids in the area of highest turbulence created by the mixer, which is preferably at the bottom of the mixing tank.
In a preferred embodiment of the invention, an essentially gas-free dentifrice paste is obtained in accordance with the above-defined process by:
(a) introducing a liquid vehicle, which usually include humectants, such as glycerol, liquefied sorbitol (generally a 70% aqueous solution) or other liquid polyols, water, gelling or stabilizing agents, which usually include gums or finely divided hydratable materials such as carboxymethylcellulose and carraghennins and/or flavoring agents and other liquid constituents for their cosmetic, therapeutic or aesthetic effect, to a mixing tank;
(b) subjecting the liquid vehicle to a continuous vacuum and intimate mixing; and
(c) introducing insoluble toothpaste solids comprising a polishing agent or abrasive or mixtures thereof, such as silica xerogels, hydrated aluminas and complex alumino silicates; and/or bodying agents, such as silica aerogels or other colloidal silicas having thickening or bodying capacity; into the bottom of said mixing tank while maintaining said vacuum and intimate mixing;
said vacuum being of sufficient level to draw the insoluble toothpaste solids into the liquid vehicle, preferably in the area of highest turbulence at the bottom of the tank.
Once the foregoing homogeneous essentially gas-free dentifrice paste is prepared, which can generally be referred to as a base paste, various other classes of ingredients may be added to finalize the toothpaste product, which generally include a mixture of humectant, such as a polyhydric alcohol, and surface active agent, and any other flavoring agents or ingredients that will finalize the desired toothpaste product. The mixture of humectant and surface active agent solution is degassed and may be added directly to the base paste in the mixing tank, but is preferably degassed and added to the base paste in a closed system downstream of the mixing tank. The surfactant/humectant solution can be made essentially gas free by heating it at atmospheric pressure or under vacuum, or by vacuum degassing. In accordance with the preferred method, the base paste and surfactant solution is withdrawn from their respective containers, and combined into one mixture in a closed system under pressure in the absence of air. By mixing the base paste and detergent solution under pressure in a closed system, entrainment of further air into the toothpaste ingredients is prevented.
Finally, any other essentially gas-free flavoring agents or other constituents may be added to the combined mixtures while pressure and the absence of air is maintained.